1. Field of the Invention
The present invention relates to an image pickup apparatus converting an image into an electric signal, and more particularly to a radiation image pickup apparatus detecting a radiation such as an X-ray and a γ-ray. The radiation image pickup apparatus is applied to a medical image diagnostic apparatus, a nondestructive examination apparatus, an analyzer using a radiation, and the like.
2. Description of Related Art
Conventionally, the radiographing methods used by a medical image diagnosis are roughly classified into general radiography obtaining a still image and radioscopy radiography obtaining a moving image. Each radiographing method is selected including a radiographing apparatus as the need arises.
As the general radiography, i.e. a method of obtaining a still image, there is a method of using a screen and film system (hereinafter abbreviated to S/F), which combines a phosphor screen and a film, and fixing the film after exposing and developing the film. Alternatively, a method (computed radiography: hereinafter abbreviated to CR) of recording a radiation image on a photostimulable phosphor as a latent image, and, after that, of scanning the photostimulable phosphor with a laser to read output optical output information with a sensor is general. However, both of the methods severally have a defect such that the work flow for obtaining a radiation image is troublesome. Moreover, although the digitization of a radiation image is indirectly possible in both of the methods, both the methods lack immediacy. Furthermore, if the digitized environments of computer tomography (CT), magnetic resonance imaging (MRI) and the like, which are used by other medical image diagnoses, are taken into consideration, both of the methods cannot be said to be in a situation of having the sufficient consistency with CT, MRI and the like.
Moreover, an image intensifier (hereinafter abbreviated to I. I) using electronic tubes are mainly used for the radioscopy radiography, i.e. for a moving image. However, because the I. I uses the electronic tubes, the apparatus of the I. I becomes large in scale. Besides, the I. I has a small visual field region yet, namely the detection area thereof is small, and making the detection area to be a large one is anxious in the medical image diagnosis field. Furthermore, from the problem on the configuration of the apparatus, an obtained moving image has much crosstalk, and the improvement to a clear image is expected.
On the other hand, now that the progress of the liquid crystal thin film transistor (TFT) technology and the improvement of the information infrastructure are replete, a flat-panel detector (hereinafter abbreviated to FPD) has been proposed in Japanese Patent Laid-Open Publication No. H08-116044 and the like as a radiation image pickup apparatus combining a sensor array composed of photoelectric conversion elements using non-single crystalline silicon such as amorphous silicone (hereinafter abbreviated to a-Si) and switching TFT's, with phosphor converting a radiation into visible light or the like. By such a FPD, the possibility of digitization of a radiation image having a large area is coming Up.
The FPD can read a radiation image in an instant, and can display it on a display in an instant. Moreover, the image can be directly taken out as digital information. Consequently, the FPD has a feature such that the FPD is convenient to treat image data by saving, working and transferring the image data. Moreover, although various characteristics such as sensitivity depend on radiographing conditions, it is confirmed that the FPD has the various characteristics which are equal or more as compared with those of the conventional S/F system radiographing method and the CR radiographing method.
FIG. 16 is a schematic equivalent circuit diagram of the FPD.
In the diagram, a reference numeral 101 denotes a photoelectric conversion element unit. A reference numeral 102 denotes a transfer TFT. A reference numeral 103 denotes transfer TFT drive wiring. A reference numeral 104 denotes a signal line. A reference numeral 105 denotes sensor bias wiring. A reference numeral 106 denotes a signal processing circuit. A reference numeral 107 denotes a TFT drive circuit. A reference numeral 108 denotes an A/D converter.
A radiation such as an X-ray enters from the upper part of the sheet, and is converted into visible light by not-shown phosphor. The converted light is further converted into a charge by the photoelectric conversion element unit 101, and is accumulated in the photoelectric conversion element unit 101. After that, the TFT drive circuit 107 makes the transfer TFT 102 operate through TFT drive wiring to transfer the accumulated charge to the signal line 104, and then the signal processing circuit 106 processes the transferred charge. Furthermore, the A/D converter 108 performs the A/D conversion of the processed charge to output the converted digital data.
Basically, the element configuration mentioned above is general, and in particular, as the photoelectric conversion element, various elements such as a PIN type photodiode or an MIS type photo-sensor, which the present inventors adopt, have been proposed.
FIG. 17 is a schematic plan view of a pixel in the case where the MIS type photo-sensor is adopted as the photoelectric conversion element.
A reference numeral 201 denotes an MIS type photo-sensor. A reference numeral 202 denotes a transfer TFT. A reference numeral 203 denotes transfer TFT drive wiring. A reference numeral 204 denotes a signal line. A reference numeral 205 denotes sensor bias wiring. A reference numeral 211 denotes a transfer TFT gate electrode. A reference numeral 212 denotes transfer TFT source and drain electrodes (hereinafter abbreviated to SD electrodes). A reference numeral 213 denotes a contact hole.
Moreover, FIG. 18 shows a sectional view of schematically arranged each element in the pixel shown in FIG. 17. A reference numeral 301 denotes an insulating substrate such as a glass substrate. A reference numeral 302 denotes the transfer TFT drive wiring. A reference numeral 303 denotes the lower electrode of an MIS type photo-sensor. A reference numeral 304 denotes a transfer TFT gate electrode. A reference numeral 305 denotes a gate insulating film. A reference numeral 306 denotes an intrinsic a-Si film. A reference numeral 307 denotes a hole blocking layer. A reference numeral 308 denotes the sensor bias wiring. A reference numeral 309 denotes the SD electrodes of the transfer TFT. A reference numeral 310 denotes the signal line. A reference numeral 320 denotes a protective film. A reference numeral 321 denotes an organic resin layer. A reference numeral 322 denotes a phosphor layer.
As apparent from FIGS. 17 and 18, because the MIS type photo-sensor and the TFT, which is a transfer switching element, have the same layer configuration, the manufacturing method of the FPD is simple, and the FPD has an advantage in the possibility of the realization of a high yield and a low price. Moreover, the various characteristics of the FPD, such as the sensitivity thereof, are evaluated to be sufficiently satisfiable. Accordingly, as an apparatus to be used for general radiography now, the FPD mentioned above has resulted in being adopted in place of the conventional S/F method and the CR method.
However, although the FPD mentioned above has a large area and has achieved the complete digitization to be in the situation of being beginning to be mainly used for the general radiography, the FDP is in the situation of being insufficient in reading rate yet for the radioscopy radiography.
FIG. 19 is an equivalent circuit diagram of one bit of the FPD using the MIS type photo-sensor.
In the diagram, a reference character C1 denotes the resultant capacity of the MIS type photo-sensor. A reference character C2 denotes the parasitic capacity formed on the signal line. A reference character Vs denotes sensor bias potential. A reference character Vr denotes sensor reset potential. A reference character SW1 denotes a Vs/Vr selector switch of the MIS type photo-sensor. A reference character SW2 denotes an ON/OFF selector switch of the transfer TFT. A reference character SW3 denotes a signal line reset switch. A reference character Vout denotes an output voltage.
The potential Vs is given to the MIS type photo-sensor through the switch SW1 as the bias potential in order that the semiconductor layer may be depleted. In this state, when converted light from the phosphor enters the semiconductor layer, the positive charge prevented by the hole blocking layer is accumulated in the a-Si layer, and a potential difference Vt is generated. After that, an ON voltage of the transfer TFT is applied to the transfer TFT through the switch SW2, and the voltage difference Vt is output as the output voltage Vout. The output voltage Vout is read by a not-shown readout circuit, and, after that, the signal wiring is reset by the switch SW3. Thus, the readout is sequentially performed.
According to the drive method, the readout of the whole frame is completed by turning on the transfer TFT's one by one on every line (row). After that, the reset potential Vr is applied to the MIS type photo-sensor through the switch SW1 to perform the reset thereof. Then, the bias potential Vs is again applied, and the MIS type photo-sensor enters the image reading accumulation operation.
For example, in a FPD having a pixel size of 160 μm and a pixel region 43 cm×43 cm, the resultant capacity C1 of a MIS type photo-sensor is about 1 pf, and the parasitic capacity C2 thereof is about 50 pf. At this time, about 2% of transfer remainder is generated in the resultant capacity C1 as a charge share at the time of the transfer. Accordingly, at the time of radiography, the reset operation mentioned above becomes indispensable in order to maintain image quality. Specifically, it is needed for about ten msec to several tens msec per frame to perform the reset operation. Naturally, the time is dependent on reset conditions. In other words, in order to realize the radioscopy radiography which needs to perform the high speed reading of 30 frames per second (hereinafter abbreviated to 30 FPS) or more, for a question for 1 second, it is necessary to perform reading processing, reset processing and the like of all lines in one frame of 33 msec (30 FPS).
FIG. 20 is a schematic view illustrating the drive method.
In the view, a reference character T1 denotes the processing time of reading one line and the like. A reference character T2 denotes the processing time reading all lines and the like. A reference character T3 denotes the processing time of a reset and the like. A reference character T denotes the processing time of one frame.
As mentioned above, if the one-frame processing time T is required to be 33 msec or less and the processing time of a reset and the like T3 is supposed to be 15 msec, then the processing time T2 becomes 18 msec. If it is supposed that 1,500 lines are read, the processing time of reading and the like assigned to one line T1 becomes 12 μsec. Moreover, if a radiation shooting exposure time, i.e. a sensor accumulation time is included, the processing time of reading and the like T1 is further restricted. Consequently, it is necessary to improve the performance of the transfer TFT, and it leads to enlarging the size of the TFT at the sacrifice of the open area ratio thereof. Conversely, many problems, such as a fall of sensitivity, the deterioration of image quality, and an increase of a radiation dose, occur frequently.
That is, a high speed moving image drive and image quality are in a relation of a trade-off, and it is the situation that a high-definition high-speed moving image cannot be obtained under the present conditions.
In the above, although the case where the MIS type photo-sensor is used has been described, fundamentally, also in the case of using the PIN type photodiodes, processing time of a reset and the like is the problem of a moving image drive.
Accordingly, Japanese Patent Publication No. 2003-218339 discloses a FPD having a plurality of pixels, each including a conversion element converting a radiation into an electric signal, a signal transferring element connected to the conversion element, a resetting element applying predetermined potential to the conversion element to perform a reset operation, wherein both of the signal transferring element and the resetting element are connected to the pixel electrode of the conversion element. The disclosure aims to improve the sensor reset system in the conventional FPD and the open area ratio besides obtaining a large area and complete digitization not only in the general radiography but also in the radioscopy radiography to realize the secure high speed moving image reading, and thereby to provide a highly reliable radiation image pickup apparatus, in particular, to provide the optimum arrangement of sensor reset switches.
In the apparatus disclosed in Japanese Patent Publication No. 2003-218339, a reset is performed every line of a plurality of pixels; it became possible to reset the sensors of the lines which have already read during the readout of one line; the open area ratio is improved; and consequently a high speed moving image drive is realized. However, it is further desired to improve the open area ratio and to realize the improvement of the functions such as a secure high speed moving image drive at a low price.